The present invention relates to a method and apparatus for granulating material and more particularly for granulating plastic and metal articles.
Plastic granulators are used to fragmentize piece scrap or waste plastic material resulting from the production of various articles such that granulated pieces can be recycled into article production operations. Similarly, waste from molding processes are granulated prior to shipment and reprocessing. Efficient granulation requires that large quantities of scrap material be gravity fed into an apparatus and uniform compact granulate exit the apparatus.
One type of granulator uses a two-stage cutting process to successively coarse cut and granulate plastic. Often, a two-stage granulator requires the use of a screen prior to material discharge from the apparatus to assure granulate uniformity. U.S. Pat. Nos. 4,151,960; 4,377,261 and 5,402,948 are representative of two-stage granulators using a screen. Access to the screen is generally obtained by physically removing portions of the granulating apparatus resulting in operational downtime. Screen cleaning is periodically necessary to remove debris clogging the screen mesh.
Existing two-stage granulators often utilize more than two rotating shafts in order to operate a two-stage cutting process. U.S. Pat. Nos. 1,826,891; 4,750,678 and 5,143,307 are representative of two-stage granulators using more than two shafts. The synchronization in torque driving of interworking shafts requires comparatively complex gearing to adequately control the results in inefficient operation and both stages are not being taxed equally.
Existing two-stage granulators typically operate at speeds of between 50 and about 1000 rpms. Such high speed operation consumes considerable power, and presents unnecessary safety and maintenance demands on granulator operation. Thus, there exists a need for a two-stage granulator operating with two shafts at low speed and independent of screens.
Another type of granulator uses a single shaft having interspersed coarse cutters and fine cutters operating at about 30 rpm. U.S. Pat. No. 4,580,733 is representative of this design. The efficiency of such a single stage design is limited by the considerable torque needed to turn the unbalanced shaft and the limited throughput associated with fine cutters having to grind coarse material. Thus, there exists a need for a granulator cutter assembly that promotes uniform cutting torque and high throughput.